Mike Bloomberg has a big problem when he starts making John Liu look mayoral.

It started when Mike went off last week, praising his signature initiative to turn city streets into a car-free haven for bicycles. “I don’t think anybody would have the nerve to roll back the bicycle lanes,” he said.

But the city comptroller, who has never been accused of a lack of nerve, seems eager to take Bloomberg on here. At a recent mayoral forum, Liu said bluntly that the bike lanes should never have been installed in the first place. And he’s been sounding the alarm about what will happen when Bloomberg’s next big plan — the bike-share program — hits the streets.

This program begins on Memorial Day. It will mean 6,000 rental bicycles dumped at hundreds of kiosks around Manhattan and Brooklyn. And that’s just the start, because plans are to expand the program to 10,000 bikes in three boroughs. Considering how cyclists already flout basic traffic laws in ways that make seasoned cabbies blush, it’s a scary thought.

But the biggest problem with the city’s bike lanes is that the NYPD has neither the manpower nor the funding to tackle the challenge of enforcing the rules of the road when 10,000 cyclists suddenly start hitting the streets around town.

We’ll have to wait for the election to see if Mike’s right in predicting that his successor won’t have the nerve to end the bike lanes. But for New Yorkers who every day see bikers run through traffic lights and stop signs, the one certainty is that it’s not going to get better — because the city has no real way to enforce the rules of the road.